There is a demand in recent years for improvement in efficiency of fan motors mounted to air conditioners for driving blower fans, for instance, because of the trend of saving energy consumption of the air-conditioning units. There are often cases that highly efficient brushless motors are used for these fan motors in place of induction motors. And in many cases, these brushless motors are driven by inverters of Pulse Width Modulation (hereinafter referred to as “PWM method”) as the driving method.
In employment of such PWM method of driving, it is likely that a phenomenon of wavelike wear occurs on bearings rotatably supporting a motor shaft when high-frequency electrolytic corrosion (“electrolytic corrosion”) has progressed, which may eventually cause abnormal noises.
Among conventional brushless motors of this kind, one example is disclosed in Japanese Patent Unexamined Publication, No. 2004-242413. Referring now to FIG. 16 to FIG. 18, description is provided of this kind of the conventional brushless motor. FIG. 16 is a partially sectioned view showing a structure of a conventional brushless motor, FIG. 17 a front view of the brushless motor shown in FIG. 16, and FIG. 18 a model diagram representing general distribution of stray capacitances in the brushless motor shown in FIG. 16.
In FIG. 16 and FIG. 17, a stator is composed of stator winding 512 wound on stator core 511 and molded with insulation resin 513. Rotor 514 is placed inside the stator with a clearance between them. Two bearings 515 are attached to shaft 516 of rotor 514. These two bearings 515 are fixed to respective positions of insulation resin 513 of the stator. Shaft 516 is supported by two bearings 515 in a manner to keep rotor 514 rotatable.
This brushless motor also includes printed wiring board 518 having a drive circuit mounted to it. This printed wiring board 518 is fixed to a position with bracket 517 when press-fitted to the stator.
In the above structure of the conventional motors of this kind, however, no measures had ever been taken against electrolytic corrosion. There had hence been problems such as those described below.
In other words, the above structure of the conventional motors carries stray capacitances C1 to C7 and so on, among the individual structural components due to its configuration, as shown in FIG. 18. When stator winding 512 of this conventional motor is driven by an inverter of the PWM method, through currents flow through the stray capacitances C1 to C7 and the like. These currents produce differences in electric potential among the individual structural components, and they consequently cause electrolytic corrosion under certain conditions.
The conditions especially known to help develop the electrolytic corrosion include a case in which the motor is operated for a long duration of time without large variations in the rotating speed when temperature is comparatively low in a region where a voltage supplied to the motor is high (e.g., the region with commercial power supply of 240V). Described below is one example of the mechanisms that produce electrolytic corrosion.
When stator winding 512 is driven by an inverter of the PWM method, a high-frequency circulating current flows in a loop along the stray capacitances C1 to C7 of the individual structural components, from stator core 511 through stator winding 512, printed wiring board 518, bracket 517, bearings 515, shaft 516, rotor 514, and returns to stator core 511. This current causes a phenomenon of electric discharge attributable to a local dielectric breakdown inside bearings 515 if a film of grease serving the lubricant breaks off or decreases in thickness during this moment, and forms tiny discharge marks on rolling surfaces of bearings 515. If the above phenomenon continues over a long period of time, it may eventually result in electrolytic corrosion. Presence or absence of this discharge phenomenon is closely related to a value of voltage appearing across each of stray capacitances C3 and C6, or bearings 515, when divided by C1 through C7. Here, a voltage source “Vdc” shown in the figure represents a voltage applied to printed wiring board 518.
This phenomenon of electrolytic corrosion roughens the rolling surfaces of bearings 515, and eventually leads the motor to generate abnormal noises when repeated until the surfaces are worn into wavelike form. One example that shall be noted among the measures to prevent this phenomenon is a method of using bearings provided with balls made of ceramic, or non-conductive material, between inner rings and outer rings. However, the bearings provided with ceramic balls are very expensive, and they have not as yet come to wide use in mass production.
Other methods include use of electrically conductive grease for the bearings, grounding the stator core, lowering a carrier frequency of the PWM driver, and the like. None of the above methods is entirely satisfactory, however, in view of the reliability, cost, quality and convenience of use, and they have not as yet been adopted for practical use. The measures to prevent electrolytic corrosion thus entail an increase in cost of the bearings and materials of the motor, and a decrease in the convenience of use, as described above.